Member postings for Colin Whittaker

Raph,

I'm going to limit my answer to induction motors (there are also lots of DC/Universal brushed and brushless motors out there).

To start an induction motor more than one phase is needed. This is either supplied from using a three phase supply or a phasing capacitor on a second coil driven by a single phase supply. (Just realised you can have shaded pole single phase motors but they typically only work in fractional horsepowers.)

Because the induction motor only needs more than one phase to start the starting capacitor and coil are sometimes disconnected with a centrifugal switch. This disconnection allows the starting capacitor and coil to be significantly derated.

The advantage of a three phase induction motor is the absence of failure prone capacitors and centrifugal switches and the ability to vary the frequency of a VFD to vary the speed. The disadvantage of a three phase induction motor is the need for a three phase power supply that is not found in the majority of houses or the purchase of a VFD.

Running a single phase induction motor on an inverter? Obviously if the inverter is running at 50Hz and can supply the starting current (maybe double the full load current) then, yes. Using the inverter to change the frequency would be an interesting experiment. Lowering the frequency would increase the capacitor's impedance and reduce the current in the phasing coil meaning the motor would be relying increasingly on a single coil. Conversely, increasing the frequency would give more current through the phasing capacitor and it's coil. It would also reduce the phase shift between the main and phasing coils, the result of this reduced phase shift I cannot confidently predict.

And before anyone flames me I have no idea whether any particular inverter will run with just two of the outputs connected to a single phase motor with the third output dangling free.

Modern induction motors can handle very high temperatures before the coil insulation fails, see **LINK** Does your motor plate give an insulation rating?

Good luck, Colin

Nick,

My understanding, and it is some 40+ years out of date, is that to a first order approximation torque is proportional to current at small values of slip. Lowering the AC power frequency allows more current to flow giving more available torque at the same time it reduces the rotational velocity for the same value of slip.

For a conventionally wired induction motor operating at full power with the steel core close to saturating the effect of reducing the frequency would be to saturate the steel with a marginal increase in torque and a large increase in motor current that will rapidly over heat the motor. Because "Not" is working with a 430V motor driven with 230V the steel is nowhere near saturating and so this unusual constant power behaviour with variable frequency would be observed.

Best regards, Colin

P.S. You may find the following link on Induction Motor torque of interest, **LINK**

Not,

The scaling factor of 58% is pretty good approximation to the reduced power and torque. Speed and efficiency will essentially be left unchanged.

If 58% is too high for you then it becomes a lot more difficult to adjust the power down further. Reducing the VFD frequency reduces the speed but increases the current and torque. As the power is proportional to the product of frequency and torque the power remains pretty well constant. Increasing the frequency sees the opposite with the power again left unchanged.

If you can't live with 1.16 HP then the only other passive technique to reduce the motor power down to 0.75 HP would be adding resistors into the power leads. My back of the envelope calculations suggests that three 64 ohm resistors would do the job while dissipating 80 Watts each. But this is not a route I am drawn to.

What's the load? Will it ever exceed 0.75 HP?

Colin

As Muzzer says, it's probably easier to dial things down on the variable frequency drive.

However, if you want to work in star mode then to a first order approximation I would expect the power to be reduced to 1 over root 3 or 58% of the delta configuration. Motor currents will also fall (so less torque and less heating) while the rotational speed will barely change and consequently the fan cooling will remain the same.

Time for the nostalgia moment ... Many years ago in the Western Desert of Egypt I had to pull the same trick to get a workshop 60Hz air compressor to run on the town's 50Hz power supply without tripping the over-current breaker.

The downside to these call protection schemes is that they block my Skype phone calls from overseas to my elderly mother in the UK. (Skype calls are given an arbitrary UK phone number in the UK when you initiate a call.)

I sat down with the patent for this kind of fridge to try and understand how it operates and failed completely. But as Albert Einstein was the co-author of the patent, **LINK**, I guess I shouldn't feel too bad. I would counsel you against spending too much time trying to perform power and efficiency calculations.

As an aside, I had a colleague who used a gas powered version of this fridge on a sailing boat. The angle of heel would place the gas flame heat source appropriately or not resulting in starboard tack warm beer, port tack cold beer.

Back to your question, swapping 12V with 220V heaters should present no problem.

Guru Ketan,

Thank you for that carefully worded reply. The technical level you used suits me just fine. MiSUMi Thailand looks to be a convenient local supplier, **LINK** but I will first pull things to pieces as per your advice.

Is there anything I can do to repay your kindness? Anything you need from Thailand? I'll be visisting the UK in a month or so.

Best regards, Colin

Ketan,

Sorry, typo on my bearing size, it should read 35 x 62 x 14 mm.

The following link is the nearest I can find to a replacement, **LINK**

I'm not looking to save a few pennies here so guidance on a more expensive better quality replacement would be appreciated.

Colin

Ketan,

Sorry for the tardy response, I can't really offer any excuse except to say I'm retired and refining my procrastination.

Based on your argument it does look as though the headstock bearing I need to replace could be item 46, a 36 x 62 x 14 mm bearing.

The chuck is secured by a series of allen bolts to a round plate that looks to be integral with the shaft drive from the headstock. A second plate is secured by allen bolts to the casing of the headstock.

I'm worried about damaging things when I remove and replace the bearing. To a lessor extent I'm worried about opening it all up and then finding I have the wrong replacement part and having to wait a month while mail order does its stuff in Thailand.

Pictures follow below.

Cheers, Colin

Neil,

That's the beastie.

From my manual it has the following specifications ...

height of spindle centre 110mm

max swing diam over bed 220mm

max workpiece diameter over carriage 115mm

max length of workpiece between centres 450mm

max cutting length 420mm

spindle bore 20mm

taper of spindle bore Morse No. 3

I have looked for similar generic Chinese lathes but I seem to have something a little too special.

I guess I'm going to have to bite the bullet, savage a perfectly good allen key to fit in the gap between chuck and headstock, remove the three jaw chuck, and then cautiously go boldly.

Cheers.

I have a Model BB 22A Cutting Machine manufactured by the Beike Machine Tool Factory of Bengbu Machine Tool Industry Company.

Although my Chinese lathe is working well enough I can feel a little shake on the headstock bearing and I think I may have to change it. The documentation that came with the lathe is pretty minimal but I think the offending item is a 15 x 32 x 9 bearing. Can anyone point me towards any step by step guides for this repair job on this specific lathe? Any youtube videos?

Sam, Where is your boat? Maybe there's someone local who can help.

I'd be very careful feeding a variable frequency into the output transformers. Transformers have a frequency rating to ensure they don't saturate at the rated input voltage. If you decide to lower the input frequency then you'll need to lower the input voltage (probably in a linear fashion). If the objective is to finally drive a three phase motor with a variable frequency then it is beginning to sound complicated. It may work but it needs some careful thought to minimise the risk of an expensive bang.

Not a washing machine but a cooker, bought in the January sales in Cambridge in the mid 1990's for around 500 GBP.

Did I want the extended warranty? No thank you.

What about failures? Is this an unreliable cooker? No. OK then no extended warranty.

But the halogen hobs are very expensive when they fail. And so on for 5 minutes or more.

My slighly exasperated final response, "Can I buy this cooker without an extended warranty?"

It's now 20+ years later. For the last 10 years the cooker has been operating in an open air house kitchen in Phuket with the sea air doing it's best to encourage lots of surface rust. Several years back the digital clock failed and I had to bypass it to get power to the main oven. One of the hob switches was threatening to go intermittent last year but the problem went away. Last month a lightning strike punched a hole through a wire's insulation and burnt out a connection (now spliced).

Extended warranties are to make money for the retailer. If you can afford to carry the risk then you'll normally make money out of the deal.

Please excuse the telling of this off topic tale; my traditional family audience now regards this story with ill disguised boredom.

Edited By Colin Whittaker on 29/09/2017 04:07:49

I've mentioned it elsewhere on this forum but you can check out the MSDS sheet for table salt, aka sodium chloride. It's apparently deserving of filter masks and goggles. The question becomes did the same lawyers who peverted the risks of working with table salt also write the safe handling requirements of uranium and other nasties? And if so then what really are the risks?

https://www.google.co.th/search?q=msds+sodium+chloride&oq=msds+sodium+chloride&aqs=chrome..69i57j0l5.7815j0j4&sourceid=chrome&ie=UTF-8

Colin

PDP11! That's modern crap. Back around 1980 I was making punched tapes for a Marconi Elliot 18bit computer used on British Railway's Healey Mills Train Describer. To make it fun I was writing machine code to do this on an HP 16bit ferrite core memory mini-computer. And then the Union Official complained about the punched tape drive's noise in the office keeping their members awake after lunch and I was promptly shut down.

That's one of the reasons I ended up in the Oil Field.

MSDS sheets used to be a useful source of information, engineer to engineer, but then the lawyers got in on the game and insisted on over the top arse covering. As a reality check take a look at an MSDS data sheet for table salt aka sodium chloride. The protective equipment needed for a trip to the restaurant is quite impressive.

If I was to build a clock using ball bearings instead of the usual burnished steel arbor in broached brass hole technology then would I still use brass plates for the frame?

Would steel or aluminium work just as well? Would it all come down to what looks pretty?

(This is not a question about the material to use for clock wheels.)

Thanks, Colin

Rob,

With your machine code grounding you should find it all pretty straightforward. As for the kit, is this too expensive, https://store.arduino.cc/usa/arduino-starter-kit?

Cheers, Colin

Andrew,

if the governor is using gravity then some fraction of the force m.g will be balancing the governor force m.r.w^2.

If the governor is using springs then the mass will no longer cancel out on both sides and your arguement above holds.

Cheers, Colin